As the Governments’ agency responsible for safeguarding Scotland’s built heritage, Historic Scotland maintains some 330monuments in the care of the Secretary of State for Scotland. In the past, the control of biological activity and plant growths on the historic masonry of these sites was routinely carried out using a variety of “approved” treatments. However, there was some disquiet expressed that these applications may have been damaging the physical fabric to which they were applied. At the same time, there was a developing Health and Safety awareness in the general use of such chemicals. As a result, some 10 years ago, the treatments were stopped, pending a programme of detailed investigation into the need for them, and their effects.

In a related area, there was a growing concern over outbreaks of algal growths, which were occurring on many of Scotland’s stone buildings cleaned in the extensive “facelift” programme of environmental improvements carried out since the 1960s. This “clean to green” phenomenon inevitably provoked the use of other untested masonry biocides and chemical applications.

Responding to the emerging evidence of physical loss and damage to sandstone by some stone cleaning methods. Historic Scotland and Scottish Enterprise initiated research with the Masonry Conservation Research Group (MCRG) at The Robert Gordon University, Aberdeen, in 1989. Their search findings were published in 1992 as Stone Cleaning in Scotland, followed in 1994 by Stone Cleaning: A Guide for Practitioners. This work led to a number of additional questions being raised, among them that of the “greening” of buildings by surface biological growths, and what could be done to effect control of this without creating the risk of further damage.

In August 1991, Historic Scotland commissioned the MCRG to undertake a programme of further research. The aim of this was to provide a better understanding of the mechanisms of biological growth on sandstone structures, their influence on stone decay, and the efficacy of various biocide treatments which might be applied. Building on earlier associated studies, the resulting research report. Biological Growths, Biocide Treatment, Soiling and Decay of Sandstone Buildings and Monuments in Scotland, was presented and published in 1995. The findings of that detailed study form the basis of this publication.

This Technical Advice Note has also been informed by RGU’s related PhD research work by Maureen Young, and their Engineering and Physical Sciences Research Council funded project, carried out at Historic Scotland’s Hermitage Castle, by Dr Melanie Jones and Dr Rachael Wakefield.

The 10th in the Historic Scotland TAN series, this publication is not intended to be a prescriptive document. Rather, it aims to better inform practitioners as to the range of technical issues which should be considered when faced with a building covered by surface growth, in order that individual specifications can be devised to respond more effectively to the particular factors involved.

It was written by Sonja Cameron, Dennis Urquhart, Rachael Wakefield and Maureen Young, all of The Robert Gordon University, MCRG, Aberdeen, with support from Robin Kent and Una Lee, Senior Conservation Architects, TCRE Division, Historic Scotland.

Particular thanks are due to the Macaulay Land Use Research Institute for producing and providing the scanning electron micrographs, and to all other associated individuals and organisations, that participated in, and contributed to, the research work.

INGVAL MAXWELL Director, TCRE